Insulin treatment of fat and muscle cells causes a rapid increase in glucose transport. The basis for this effect is an increase of glucose transporters of the GLUT4 type at the cell surface. This increase occurs as the result of insulin-stimulated movement of intracellular vesicles containing GLUT4 to the plasma membrane and fusion therewith, a process known as GLUT4 translocation (Watson, et al. (2004) Endocrine Rev. 25:177-204). Evidence suggests that a signaling pathway necessary for GLUT4 translocation is the one that proceeds from the insulin receptor to the activation of the protein kinase B, also referred to as Akt (Watson, et al. (2004) supra; Bae, et al. (2003) J. Biol. Chem. 278:49530-49536; Jiang, et al. (2003) Proc. Natl. Acad. U.S.A. 100:7569-7574; Katome, et al. (2003) J. Biol. Chem. 278:28312-28323). However, there is less information about the connection between Akt activation and GLUT4 translocation. A 160-kDa Akt substrate protein having the properties expected for this connection has been described (Kane, et al. (2002) J. Biol. Chem. 277:22115-22118; Sano, et al. (2003) J. Biol. Chem. 278:14599-14602). This protein, which has been designated AS160 (Akt substrate of 160-kDa), has a predicted GTPase activating protein (GAP) domain toward members of the Rab protein family.
Rabs are small ras-related GTP-binding proteins that in their GTP-bound form participate in vesicle movement and fusion (Zerial and McBride (2001) Nat. Rev. Mol. Cell. Biol. 2:107-119). The GAP for a Rab stimulates the typically slow intrinsic GTPase activity of the Rab, to generate the inactive GDP-bound form of the Rab. AS160 is phosphorylated by insulin-activated Akt suggesting that phosphorylation of AS160 inhibits its GAP activity (Sano, et al. (2003) supra); consequently, the GTP form of a Rab(s) required for GLUT4 translocation is elevated, and thus translocation is triggered. Further, insulin-stimulated GLUT4 translocation in adipocytes was blocked by expression of a mutant of AS160 lacking Akt phosphorylation sites (Sano, et al. (2002) supra). Presumably, this nonphosphorylatable mutant of AS160 continued to function as a GAP in the presence of insulin. This blockage required a functional GAP domain as the nonphosphorylatable mutant was not effective when the catalytic Arginine in the GAP domain was mutated to Lysine.